1. Field of Invention:
This invention relates generally to non-contact electro-optical scanning systems for the inspection and measurement of opaque objects having a quasi-specular surface which, when illuminated exhibits a light reflection pattern that depends on its physical structure, and more particularly to a defocusable static system of this type which functions to prevent image blurring when the object being examined is out of focus.
2. Description of the Prior Art:
Electro-optical scanning systems are known that are capable of scanning an opaque object with a light beam to produce a reflected beam which is detected to generate a video signal whose waveform represents the field of view. The present invention is applicable to the electro-optical inspection and measurement of an opaque object having a relatively smooth surface which is quasi-specular and exhibits a reflection pattern that is indicative of its physical formation.
Thus, integrated circuits are constituted by smooth substrates whose reflectivity differs from that of conductive paths and circuit elements formed thereon. The present invention is capable of illuminating the surfaces of such objects with a scanning beam to inspect the formation thereof. However, in the interest of simplicity, we shall consider by way of example an opaque object constituted by a bronze sheet of spring material having a gold inlay therein, this material being usable in making electrical connectors for telephone systems.
We shall first assume that the object being scanned is made entirely of bronze sheeting. Then the surface thereof would be uniformly reflective and the output signal of the electro-optical system would have a flat and featureless waveform, the even level of the resultant signal representing the uniformly reflective surface intercepted by the scanning beam. When, however, the object has a bronze surface interrupted by a gold inlay whose reflectivity differs from that of bronze, then the scanning system will function as an edge tracker to determine the position of the inlay by measuring the change in reflection that occurs at the edge thereof. In this instance, the waveform of the output signal will exhibit a voltage step indicative of the presence of a gold inlay in an otherwise constant signal level.
The conventional electro-optical scanning system generates in its focal plane an image of the object being scanned, this plane being perpendicular to the optical axis of the system and passing through the focus thereof. The image generated attains its maximum resolution or sharpness when the object being examined is at a predetermined distance from the lens with respect to which it is then "in focus." As the object is displaced in either axial direction from this focal distance, its image becomes blurred and therefore loses resolution.
Such image blurring or defocusing has heretofore been accepted as an inevitable concomitant of any optical scanning system of the static type. By a static optical system is meant one whose lenses have fixed positions and are therefore not axially movable to refocus the system with respect to an object being viewed which is subject to displacement relative to the focal plane.
Thus when the object to be examined is formed of a continuous strip of spring metal that is being advanced in the course of production, it is as a practical matter impossible to maintain the position of this longitudinally-moving strip exactly in the focal plane of the optical system, for the strip tends to move up and down relative to the track on which it is supported. Since the vertical displacement of the strip with respect to the focal plane is random and intermittent, one cannot use a dynamic optical system whose focus is adjustable. There is no practical way, in a high speed scanning system, by which the adjustment of focus can automatically be correlated with the changing position of the object being examined.
A similar depth-of-focus problem exists in the microcircuit industry, where microcircuit printing masks are aligned relative to the finely detailed imagery on a partially completed wafer. In this application, alignment accuracy of better than 10 microinches is required between mask and wafer. However, the mask and wafer must be kept apart, separated by an air gap of several milliinches during the alignment process in order to avoid abrading and damaging their surfaces. This separation imposes a severe depth-of-focus requirement on the alignment process which is difficult to meet with conventional technology.
The prevailing assumption that defocusing is unavoidable in a conventional static optical scanning system wherein the object position is unstable is based on the fact that a system of this type has a single field stop that defines the instantaneous field of view, the field stop having a fixed position within the optical system. Sharp images of this field stop are generated by the static optical system in defined and fixed conjugate planes. Any deviation of the viewed object surface from such a conventional conjugate focal plane unavoidably gives rise to defocus and blurring of the image, with a consequent loss in measurement accuracy.
While it is possible to compensate for image blurring by processing the waveform of the output signal of the electro-optical system, this processing must take into account the comparative brightness of neighboring points in the image. To do so entails complex and costly image data processing apparatus.